This invention relates in general to thermoplastic injection molding nozzle tips used on injection molding machines. In particular, the present invention is directed to a thermoplastic injection molding nozzle tip having predetermined temperature manipulation characteristics to effect clean cut-off of molten material.
An injection nozzle tip on an injection molding machine is in contact with a metallic mold. The mold is cooled to a relatively low temperature for the formation of a desired object from the molten material sent through the injection nozzle. As a result, the injection nozzle tip is generally cooled by the mold even though the injection nozzle tip is independently heated.
A number of problems arise from this arrangement. In particular the end of the injection nozzle tip which is in contact with the mold is cooled by the heat sink action of the mold so that this part of the injection nozzle tips tends to be lowered to a temperature much less than that of the molten material handled by the nozzle tip. Compensation for this situation is provided by a heating coil at the opposite end of the injection nozzle tip. As a result, much of the nozzle tip becomes overheated to compensate for the heat sink effects of the mold. This leads to a very uneven distribution of heat throughout the injection nozzle.
Because of massive temperature differences between the two ends of the injection nozzle, control of the temperature of the nozzle tip and the material passing therethrough becomes extremely unreliable. For example, if the nozzle tip front end is set to an appropriate temperature, thereby compensating for the heat sink operation of the mold, the other end of the injection nozzle tip must be set to such a high temperature that the molten material being handled is often burned or otherwise degraded.
Also, when overheating occurs an uncontrolled flow of resin is pushed through the injection nozzle tip, leading to undesired phenomenons such as xe2x80x9cstringingxe2x80x9d or xe2x80x9cdroolingxe2x80x9d. If, on the other hand, the nozzle tip is insufficiently heated so that the portion of the injection nozzle tip against the external mold is relatively cool, clogging of the nozzle will occur, along with the formation of large plugs.
Even minor clogging of the nozzle tip greatly increases the amount of pressure necessary to push the molten material into the mold. Insufficient pressure will cause uneven or incomplete filling of the mold by the molten material, thereby degrading the resulting product.
A number of developments have occurred in the evolution of injection molding nozzle tips, resulting in the present conventional designs. The three most common nozzle tip designs are: (a) general purpose nozzles used for both crystalizing and amorphous materials; (b) ABS reverse taper nozzles which are dedicated for use with amorphous materials; and, (c) nylon taper nozzle tips which are directed to controlling drool of free-flowing molten materials such a nylon. All three of these conventional designs are illustrated in the materials included with the Information Disclosure Statement accompanying this application. All of these designs, however improved over the years, have limitations as to their specific uses.
A general purpose nozzle tip is relatively effective for the basic conveyance of both amorphous and semi-crystalized resins. However, there is some difficulty with controlling strings, which result from materials being drawn out from the nozzle tip after the mold has been filled and the flow of resin cut off. This occurs because the mold acts as a heat sink which pulls heat from the nozzle tip at a moderately consistent rate, and due to the polymer behavior during cooling.
The tip opening area of the nozzle tip gives up the most heat due to the physical contact between the nozzle tip and the mold through the process of conduction. As a result, the plastic forms within the nozzle tip due to the action of this heat sink action of the mold. Very often, the resin cools to an unstable glass transition state which is very difficult to control if the mold sprue (at the opening of the mold) must break cleanly and be free of strings. Because the resin at the sprue opening of the mold must freeze for removal from the mold, there is a tendency of the resin to freeze at the interface between the mold and the nozzle tip. Such freezing also occurs immediately within the nozzle tip. However, beyond the sprue opening of the mold, the plastic must stay molten in order to completely fill the mold, and to be at the proper process temperature for the next molding cycle.
Strings occur when molten plastic is drawn out from the nozzle tip by an attachment to the frozen plastic at the sprue opening. The existence of strings is caused when there is not a clean freeze point or xcex94t transition at which the molten resin is cleanly separated from the frozen resin. The strings can contaminate both the mold and the injection nozzle tip itself. In particular, strings can be drawn into the open molds, degrading the product. Further, strings can contaminate the overall processing area causing a variety of different problems. Residue of strings, like frozen slugs or drool remaining within the injection nozzle tip can alter the thermal characteristics as well as the flow characteristics of that nozzle tip, thereby altering the overall performance of the nozzle tip and degrading the resulting product formed within that mold.
Various techniques are used to compensate for string formation. In one example a much smaller injection nozzle tip orifice (interfacing with the sprue opening of the mold) is used than would be recommended for a particular size of mold sprue orifice in order to reduce the contaminating effects of strings. Unfortunately, this technique causes high pressure loss. As a result, there may not be sufficient pressure to properly fill and pack the mold with the resin. Consequently, the product quality and uniformity will be substantially degraded. Further, the reduction in orifice size of the nozzle tip leads to additional undesirable freeze-off in the nozzle tip. The result is a frozen residue within the nozzle tip that compromises the nozzle tip""s performance for the next injection cycle. Also, the required pressure (to fill the mold properly) is also increased.
One expedient to control the occurrences of freeze-off and strings has been use of a cardboard buffer between the injection nozzle tip and the mold. This technique has been used as a way of moving or otherwise controlling the xcex94t point to allow a proper break between the molten resin and the frozen resin. Unfortunately, this is usually a temporary expedient, and an extremely inefficient way to use standard injection nozzle tips. Also, the cardboard rapidly degrades, causing variations between xe2x80x9cshotsxe2x80x9d of resin injection. Other expedients, such as the use of ceramic buffers between the nozzle tip and the mold sprue opening have also proved ineffective and quite inefficient.
Accordingly, even using all of the ingenuity available in modem molding processes, the conventional general purpose nozzle tip is a device in which resin flow is difficult to control, and the overall molding process using such nozzle tips ultimately becomes very inefficient.
The ABS tip was developed specifically to control stringing, an inherent property of ABS (amorphous) resins. Unfortunately, conventional models of such nozzle tips provide only marginal improvement, and can provide only an inconsistent xcex94t or break point for the mold-nozzle tip interface. This performance results in long chunks of frozen material being pulled inconsistently from the nozzle tip, or strings. Since ABS tips are designed for amorphous resins, they do not work particularly well with crystalline resins due to the enormous strings that are inherent with this type of nozzle tip design. Attempts have been made to control the strings through the use of lowering the temperature. However, this technique also causes increased plastic pressure drops which effect the finished molded product. Another problem of the ABS nozzle tip design is that it is arranged in a full taper increasing from the nozzle tip orifice (interface with the mold sprue) to the rear of the nozzle tip. Consequently, this design has a much greater pressure loss than the aforementioned general purpose injection nozzle tip.
The nylon nozzle tip is especially configured for nylon material which has a very free-flowing nature (low viscosity) inherent in the nylon resin family, a semi-crystalline polymer. Due to the low viscosity of nylon resins, a very narrow restrictive flow path is part of this nozzle design. A nylon tip is rarely used for any resin other than nylon. Due to the long, narrow, flow path, the plastic pressure losses make this type of tip all but unusable for amorphous resins. While the nylon tips are partially effective to prevent dribble or other such leakage, they are still subjected to the formation of strings and the freeze-off of large slugs deep within the injection nozzle tip.
Based on even the most well-developed and refined of the conventional injection nozzle tip designs, there are still limitations which cause severe problems in long term plastic injection molding processes. In particular, conventional injection nozzle tips are generally not capable of controlling strings or nozzle freeze-off without substantially altering the molding process, and thereby adding complications to the overall molding process. Accordingly, an injection nozzle tip is needed that will control the undesired byproducts of the injection molding process without further complicating that process.
Accordingly, it is the first object of the present invention to eliminate or otherwise control the drawbacks inherent to injection nozzle tips found in the conventional art.
A second object of the present invention is to provide an injection nozzle tip that is suitable for a variety of resin materials, including PC, ABS, PS, PE, PP, POM, acrylics, and nylon.
Another object of the present invention is to provide an injection nozzle tip that helps eliminate mold parting line damage caused by strings, thereby increasing the life of molds.
A further object of the present invention is to provide an injection nozzle tip that helps eliminate contamination and loose particulate matter on finished molded products.
An additional object of the present invention is to provide an injection nozzle tip that facilitates the reduction of scrap, and increased output for injection molding processes.
Yet another object of the present invention is to provide an injection nozzle tip that facilitates an injection molding process wherein no time is lost due to sprue break during the molding process.
Yet an additional object of the present invention is to provide an injection nozzle tip that does not require special mold opening speeds in order to break strings or other frozen detritus of the molding process.
Still a further object of the present invention is to provide an injection nozzle tip that permits the elimination of cardboard or other intermediate materials used to control freeze-off of the nozzle tip.
Yet another object of the present invention is to provide an injection nozzle tip that facilitates consistent injection pressures during the molding process.
Still a further object of the present invention is to provide an injection nozzle tip that facilitates faster injection molding cycle times when compared to the conventional nozzle tips of the same size, and servicing the same molds.
Yet an additional object of the present invention is to provide an injection nozzle tip having increased manufacturing SPC capabilities over conventional nozzle tips of the same size and serving the same molds.
Still another object of the present invention is to provide an injection nozzle tip that facilitates uniform operation from cycle to cycle throughout an injection molding run.
Yet another object of the present invention is to provide an injection nozzle tip that is less expensive to operate than conventional nozzle tips of the same size.
These and other goals and objects of the present invention are achieved by an injection molding nozzle tip arranged to conduct molten resin to an external mold. The nozzle tip includes a melt reservoir arranged to contain molten resin and a heat sink section abutting the external mold. A transition section is arranged between the reservoir section and the heat sink section. The transition section is configured to provide a point of high temperature differential whereby stringing and drooling are eliminated from the nozzle tip.
In another embodiment of the present invention a method of operating an injection molding nozzle tip is carried out to eliminate drooling and string generation. The method includes the steps of forming a heated reservoir of molten resin and forming a heat sink adjacent to an external mold to he provided with molten resin. Then, a transition section is configured between the heated reservoir and the heat sink to place the heat sink and the heated reservoir in close proximity to each other. As a result, a high temperature differential is created at the transition section, thereby eliminating drooling and string generation.